1. Field of the Invention
The present invention relates to semiconductor electron emissive materials for use in electron tubes and particularly to novel structures and processes providing a free standing thin film of GaAs as a transmission secondary electron emitter or as a photocathode layer supported on a layer of GaAlAs.
2. Description of the Prior Art
The use of GaAs as a thin film self-standing transmission electron emissive material has been known and techniques for obtaining such films include electrolytic, chemical and vapor etching processes and mechanical polishing methods which require precise control with constant careful attention during processing. The thin films may have imperfections and non-uniformities and are difficult to assemble and utilize in a vacuum tube environment. Preferential etches have been used with a GaAs/GaP structure wherein nitric acid etches GaAs but does not dissolve GaP. This process is described further in U.S. Pat. No. 3,429,756, issued Feb. 25, 1969. Nitric acid however would also etch GaAlAs. It is likewise known that hydrochloric acid preferentially etches GaAlAs while not affecting GaAs. The use of a thin monocrystalline semiconductor layer supported on a thicker transparent layer as a target window in an image tube is also shown in U.S. Pat. No. 3,579,012, issued May 18, 1971. These prior art devices however required complex processes and structures and were not useful with the combination of GaAs and GaAlAs materials which have desirable characteristics for radiation sensitive electron emissive structures such as used in electron tubes for photocathodes and electron multipliers.